For a long time it has been common practice to heat wood chips in a refiner in order to produce a mass, which can then be processed into paper or paper products. Refiners, which are often called beaters or pulpers, are used for the beating of different types of cellulose masses which may contain or be freed from lignin.
In principal there are two different types of refiners, namely so-called conical refiners and disk refiners. In the former, a cone having knives on its surface rotates, and is surrounded by a corresponding conically-shaped mantle, which is also provided with knives, in this case on its inner surface. A suspension of chips in water is then allowed to flow from the narrow end of the cone to its wider end, while the cone rotates and the chips are beaten in the beating gap which exists between the cones.
Disk refiners are possibly more commonly used, and consist principally of a stationary disk against which a corresponding disk rotates with a certain degree of play. So-called twin refiners may also be used, in which stationary disks are arranged at either side of the rotating disk. Also, so-called double-rotating or counter-rotating disk refiners are used in which both beating disks rotate counter to each other, and the material to be ground is introduced through the center of one of the disks, which is provided with spokes. Those surfaces of the disks which face each other are provided provided with knives. The material to be ground is introduced in the central part between the disks and is conveyed out towards their periphery during the beating process. To obtain efficient beating, the disks must be pressed against each other and one of the beating disks, preferably the stationary one, is therefore provided on one side with a pressure arrangement, preferably of a hydraulic type, so that it can be pressed against the other, or rotating disk. The resulting pressure which, for example, is suitable for beating chips, is up to a few bars, for example 6 bars, and the temperature is, in such a case, between about 170.degree. C. and 180.degree. C. This means that the water is at its boiling point, and saturated steam is preferably created to some extent. It is also possible to carry out the beating at higher temperatures, whereby super-heated steam is created.
The process which occurs in a disk beater is schematically illustrated in FIG. 1. This figure represents a sectional view of a stationary grinding disk 1 against which a rotating grinding disk 2 is pressed, using a suitable pressure. The material to be ground 3 arrives at the center and is conveyed outwardly against the periphery past the arrows shown thereon.
The temperature of the material which is thus beaten rises from the center outwardly towards the periphery. This occurs as a matter of course, due to the inner friction which arises between the beating disks and the chips/mass suspension introduced therebetween. This friction is small at the center where the grinding disks have a small peripheral velocity and a large beating play, but it rises outwardly with a rising peripheral velocity of the grinding disk 2 and decreased grinding play. It has therefore always been believed that the highest temperature is obtained closer to the peripheral part of the beating disks in an area which is denoted by reference numeral 4 in the Figure. However, it has been shown that the maximal temperature is in the area which is marked with reference numeral 5 for refiners which are fed with chips, and centered to 6 for refiners which are fed with pulp; i.e., substantially further towards the center.
FIG. 2 shows two curves which plot the temperature against the radius from the center of the beating disks. The curve 1 has been drafted for a maximum pressure of 6 bars between the beating disks 1 and 2, corresponding to a highest temperature of from about 170.degree. C. to 180.degree. C., the so-called pressure peak, which in this case is near the center. The curve 2 has been drafted for a pulp refinery, and accordingly the pressure peak and the temperature peak have in this case been displaced to the right; i.e., closer to the periphery of the grinding disks. The curve 2 relates primarily to a beating material which has been previously beaten, and the pattern of the beating disks is therefore somewhat finer.
Swedish Patent No. 407,952 discloses a method and apparatus for controlling the beating in a refiner including temperature and pressure gauges supplying signals to a computer which conducts the beating process. The signal values are, however, not used for the purpose of controlling the quality of different parameters of the mass, or of paper produced from the mass.
The above discussion represents an example of the fact that the beating conditions in a refiner can vary greatly. These conditions, in turn, influence the quality of different parameters of the finished beaten pulp, or of the paper which is produced from this pulp. Accordingly, there is a considerable need to be able to influence the beating process so that the desired parameters can be obtained. The physical properties which influence the beating conditions are the pressure in the beating zone, the temperature in the beating zone, and the concentration of chips and fibers. If the quality parameters are to be influenced, these physical conditions must accordingly be controlled; i.e., the feed of chips and water, the temperature of the water, and the pressure; i.e., the hydraulic pressure which is applied against one of the beating disks. No one has previously been able to predict how these factors cooperate and how they can be regulated in order to produce a desired result.